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Full text of "The Cambridge Natural History"

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atlas, and articulates with the skull. The most remarkable
fact about this bone (shared, however, by lower Vertebrates)
is that its centrum is detached from it and attached to the
next vertebra, in connexion with which it will be referred

jpia. 7.—Human atlas  (young),  showing  de-       Fio.   8.—Inferior  surface   of   atlas   of

velopment.    x^.    as? Articular surface for            Dog.     x J.     snr  ForjuTien  for  lirst

occiput ;   <7, groove for lirst spinal  nerve            spinal    nerve ;      -w,   vertebrartttrial

and vertebral artery;  %«,  inferior arch;            canal.     (From Flower's OstevtiH/y.)
tt  transverse   process.      (From   Flower's

to immediately." The whole bone thus gets a ring-like form,
and the salient processes of other vertebrae are but little de-
veloped, with the exception of
the transverse processes, which
are wide and wing - like. In
many Marsupials, such as the
Wombat and Kangaroo, the arch
of the atlas is open below, there
. 9.—Atlas of Kangaroo. . . . (From being no centre of ossification.

Parker and Haswell*s ZoologyJ)            T         ^                   t              rnr    T

JJ J         In   others,   such   as   Thylwvrwts,

there  is  a distinct   nodule  of bone  in this  situation  not   con-
crescent with the rest of the arch.

The second vertebra, which is known as the axis or epi-
stropheus, is a compound structure, the anterior " odontoid process,"
which fits into the ring of the atlas, being iii reality the
detached centrum of that vertebra.1 It is a curious fact about
that process that it has independently become spoon-shaped in
two divisions of Ungulates; that it has become so seems to be
shown by the fact that in the earlier types of both it has the
simple peg-like form, which is the prevailing form. The cervical

1 Its independence from the cpistropheiis  is emphasised in Monotremes and
iiae* Marsupials by its late fusion with that vertebra.